in the general purposes for which investigations are made. Their
methods of investigation are their own--originate with themselves, and are
carried out by themselves. But in relation to the scientific operations of
such a government institution, there is an unofficial authority which,
though not immediately felt, ultimately steps in to approve or condemn,
viz., the body of scientific men of the country; and though their
authority is not exercised antecedently and at every stage of the work,
yet it is so potent that no national scientific institution can grow and
prosper without their approval, but must sooner or later fall and perish
unless sustained by their strong influence.

As director of the Geological Survey, I deeply realize that I owe
allegiance to the scientific men of the country, and for this reason I
desire to present to the National Academy of Sciences the organization and
plan of operations of the Survey.

A TOPOGRAPHIC MAP OF THE UNITED STATUS.

Sound geologic research is based on geography. Without a good topographic
map geology cannot even be thoroughly studied, and the publication of the
results of geologic investigation is very imperfect without a good map;
but with a good map thorough investigation and simple, intelligible
publication become possible. Impelled by these considerations, the Survey
is making a topographic map of the United States. The geographic basis of
this map is a trigonometric survey by which datum points are established
throughout the country; that is, base-lines are measured and a
triangulation extended therefrom. This trigonometric work is executed on a
scale only sufficiently refined for map-making purposes, and will not be
directly useful for geodetic purposes in determining the figure of the
earth. The hypsometric work is based upon the railroad levels of the
country. Throughout the greater part of the country, there is a system of
railroad lines, constituting a net-work. The levels or profiles of these
roads have been established with reasonable accuracy, and as they cross
each other at a multiplicity of points, a system of checks is afforded, so
that the railroad surface of the country can be determined therefrom with
all the accuracy necessary for the most refined and elaborate topographic
maps. From such a hypsometric basis the reliefs for the whole country are
determined, by running lines of levels, by trigonometric construction, and
in mountainous regions by barometr